1. Field of the Invention
The present invention relates to electrophotographic processes and, more particularly, to an apparatus and a method for developing images in an electrophotographic process.
2. Description of the Related Art
Most computer systems, and most particularly small or "personal" computer systems, have been making increasing use of colors in the display of information. The increasing use of color in personal computers reflects both the more ready availability of low price color displays and the more powerful and faster microprocessors that are used in personal computers. The technology for providing printed color output, i.e., "hard copy" output such as color printouts on paper, plastic or other materials, has not kept pace with the technology for the color display of information. Presently, the options available for color hard copy output either do not present sufficiently high quality output or are undesirably expensive for home or small office use. Examples of this conventional color output technology include ink jet printing, whether using liquid or solid inks, as well as a few different implementations of color electrophotographic printing. Ink jet printing is comparatively inexpensive, at least when using liquid inks, but tends to be slow and it is difficult to obtain acceptably high quality output using liquid ink jet printing technology.
Color electrophotographic printing is commercially available, but tends to be expensive and slow. For example, multipass color electrophotographic printing is a process by which multiple photoconductor exposures and multiple developing processes are used to create a multicolor image on the surface of a paper sheet. In essence, conventional multipass color electrophotographic printing consists of repeated application of single color or monochrome electrophotographic printing processes using different colors for each successive application or pass through the multipass printer. Traditional monochrome (black and white) electrophotographic printing forms an image on the optically active surface of a photoconductor by exposing the photoconductor using a laser or an equivalent high intensity light source. Before exposure, a uniform charge distribution is provided over the surface of the photoconductor and, after exposure, a charge pattern corresponding to the exposure image to be printed is present on the surface of the photoconductor. The latent image corresponding to the charge pattern on the surface of the photoconductor is converted to a physical image by a developer which adheres charged toner particles to the charge pattern on the photoconductor in a pattern corresponding to the latent image. The toner image is transferred onto a paper sheet using an electrostatic transfer process and then fusing is performed to fix the toner image on the paper sheet. In a multipass color laser printer or other similar electrophotographic apparatus, this printing process would be repeated several times.
FIG. 1 illustrates a conventional, multipass color electrophotographic apparatus which is assumed to be a laser printer for the purposes of this discussion. The multipass laser printer of FIG. 1 includes a photoconductive drum 10, a charger 14 for creating a uniform charge distribution on the surface of the photoconductive drum 10, a laser beam scanning unit 16 for exposing the surface of the photoconductive drum with an optical image, a developing device 20 including a plurality of single color developing units 22, 24, 26, 28 for developing a latent image, a transfer charger 30 for applying a transfer electrical field, and a fuser 40 for fixing an image onto a recording medium 32 such as a paper sheet. The photoconductive drum 10 is generally a metal cylinder covered by an optically active material 12 known as the photoconductor. The photoconductor generally is highly insulative in the dark while developing a substantial level of conductivity under illumination. Thus, the photoconductor 12 can hold a charge on its surface in the dark, but charge on the surface of the photoconductor is discharged under illumination.
In operation, a uniform charge is applied to the surface of the photoconductor 12 at the beginning of each pass of the multipass color printing process. Charging of the photoconductor surface is accomplished with charger 14, which typically uses corona charging or a similar technique to provide charge to the surface of the photoconductor 12. After the charging operation, the photoconductor 12 has on its surface a uniform charge distribution corresponding to a voltage of .+-.600.about..+-.800 V. When the photoconductor 12 is exposed by the laser beam scanning unit 16, a laser beam 18 directed by the scanning unit 16 illuminates a specified area of the photoconductor 12 in accordance with an image modulation pattern generated by a controller (not shown). The voltage on the portions of the photoconductor 12 illuminated by the laser beam 18 is discharged to approximately 0.about..+-.150 V.
Multipass color laser printing is accomplished by successively forming on the surface of the photoconductor 12 successive monochrome images so that, when all of the monochrome images are combined together on the photoconductor 12, the combined image provides an acceptable color image. Typical multipass color electrophotographic strategies use four printing passes, with each successive pass applying a different optical image to the photoconductor corresponding to a different monochrome image component. Each successive image is developed after the exposure portion of the pass with a developer having the appropriate color of toner corresponding to that monochrome portion of the image. To effect this strategy, it is necessary to provide four different developing units 22-28 as shown in FIG. 1 having four distinct colors of toner to be applied in successive ones of four different passes. Thus, four developing units 22-28 corresponding to yellow (Y) toner, magenta (M) toner, cyan (C) toner and black (K) toner respectively are provided for the FIG. 1 printers. The reproduced image is therefore made up of a plurality of colors applied in varying concentrations to achieve various gray levels.
In a first pass of the multicolor printing process of FIG. 1, the laser beam scanning unit 16 exposes the surface of the photoconductor with modulated laser light 18 to create a first latent image component corresponding to the first monochrome component of the image to be printed. After the photoconductor 12 is exposed with the first latent image component, the first component of the image pattern is developed using a first developer 22, described in greater detail below, to provide a first color of toner to the surface of the photoconductor. After the first monochrome component of the color image to be printed has been provided on the surface of the photoconductor, a second pass is performed to provide a second monochrome component of the color image to be printed. The photoconductor 12 on the drum is charged to provide a new uniform charge distribution on the photoconductor. The laser beam scanning unit 16 then scans the laser beam 18 over the surface of the photoconductor to expose the photoconductor 12 with a second latent image component. A second color of toner is applied by the second developing unit 24 so that it adheres to the photoconductor 12 in a pattern corresponding to the second latent image component and overlies the first toner image. This process is repeated for the third and fourth components of the image, using the third and fourth developing units 26 and 28, respectively, to provide four different overlaid monochrome toner images on the photoconductor. The four color toner image is then transferred onto the surface of a recording medium 32 such as a paper sheet at the transfer charger 30 and the toner image is fused to the recording medium 32 at fuser 40. To accomplish fusing, the recording medium 32 is passed between the heating roller 42 and the pressing roller 44 that make up the fuser 40. A heater, such as a halogen lamp, is provided in the heating roller 42 to heat a surface of the roller to a predetermined high temperature sufficient to at least soften the developer, when combined with the pressure applied by the pressure roller 44. The high temperature and pressure between these two rollers cause the toner to melt and to be fixed onto the recording medium 32, thereby forming a color image on the recording medium.
For the four pass color laser printer illustrated in FIG. 1, four developing processes are needed to create an image. Because the FIG. 1 printer essentially requires four complete and independent printing processes to create an image, the FIG. 1 printer is about four times as slow as a conventional monochrome laser printer. As such, the color image reproduction rate for the FIG. 1 printer is generally unacceptably slow.
FIG. 2 is a schematic view showing a different conventional implementation of a color electrophotographic apparatus. The "tandem" color electrophotographic apparatus of FIG. 2 is similar to the apparatus of FIG. 1 in that the printed color image is built up over the course of repeated distinct monochrome printing processes. For the FIG. 2 apparatus, however, the four distinct printing processes are performed in series on four distinct drums 10 having associated chargers 14, laser beam scanning units 16, four distinct developers 22, 24, 26, 28 carrying four different colors of toner, and four transfer chargers 30. Operation of the FIG. 2 apparatus is generally similar to that of the FIG. 1 apparatus, with the primary exception that each component of the latent image is formed on different photoconductive drums by dedicated laser beam scanning units in the FIG. 2 apparatus. Like elements in FIG. 2 are represented by like reference numerals in FIG. 1. A first color image is formed and then transferred to a recording medium 32 from the first photoconductive drum 10, and then second, third and fourth color images are successively transferred from the second, third and fourth photoconductive drums in sequence. As illustrated, the FIG. 2 structure can accommodate a linear transport path for the recording medium 32 so that the four color components of the image can be transferred to the recording medium in a single transport operation. Thereafter, a fuser 40 fixes the resultant four component image on the recording medium 32. The apparatus of FIG. 2 is advantageous in that the reproduction rate thereof is much higher than that of the apparatus shown in FIG. 1 since the four developing processes can proceed simultaneously. The tandem color electrophotographic apparatus of FIG. 2 has undesirable characteristics, however. Although the FIG. 2 apparatus is improved over the FIG. 1 apparatus, there remains in the FIG. 2 apparatus a difficulty in obtaining a desirable level of registration between the successive images transferred onto the paper sheet or other recording medium due to the need to align four different drums with the recording medium for the four successive toner transfer operations. More importantly, the FIG. 2 apparatus can be undesirably large and expensive due to the need to provide multiple complete print stations.
A different implementation of a color electrophotographic apparatus is illustrated in FIG. 3. The illustrated color laser printer provides four color components for a printed color image while using a single pass printing operation. The FIG. 3 laser printer includes a photoconductive drum 10 with four printing stations, each including a charger 50-56, a laser beam scanning unit 58-64 and a developer 60-72, all arranged around the circumference of the drum. At the first printing station, the drum 10 is charged to an initial uniform voltage by the charger 50, the drum is exposed according to a first component of the image to provide a first latent image component, and the first latent image component is developed by the developer 66 to produce a first toner pattern on the surface of the drum. The second printing station repeats this process using a second charger 52 to reproduce a uniform charge distribution over the surface of the drum 10, including over the portions of the drum covered by the first toner image. As second toner image is created on the surface of the drum and is overlaid with the first toner image. Third and fourth color image components are created on the surface of the drum 10 as the drum rotates through the third and fourth developing stations to provide a four component, four color toner image on the surface of the drum. The four color toner image is then transferred onto the recording medium 32 by transfer charger 30, and four color image is fused onto the recording medium 32 at fuser 40.
The FIG. 3 single pass color printer provides color output at higher speeds than the multipass design illustrated in FIG. 1. As a practical matter, however, it is difficult to achieve the necessary registration of images for the FIG. 3 apparatus because of the exacting alignment accuracy required to obtain registration of the images created at successive ones of the printing stations. To obtain acceptable levels of registration for the different image components of the FIG. 3 design, it is necessary to arrange the four different laser beam scanning units 58-64 so that the laser beams trace lines on the surface of the drum that are parallel to the cylindrical axis of the drum to a very high degree. Because this alignment requires precise positioning of five objects in relation to each other in three dimensional space, it is very difficult to achieve an acceptable level of alignment, so that the FIG. 3 apparatus is not amenable to high volume manufacturing techniques. The difficulty of aligning the four laser beam paths is heightened because the lasers are typically scanned using high speed rotating polygon mirrors, with cach mirror rotating independently of all others. Small variations in mirror position will thus be magnified by the long path traced by the laser beam reflected from the mirror to the surface of the drum.
A further difficulty with the apparatus of FIG. 3 is that it tends to be large. The drum 10 must be made sufficiently large so that the four different print stations and the transfer charger 30 can be arranged about its periphery. Thus, it is difficult to make the apparatus of FIG. 3 in a small enough form factor to comfortably fit into the home and small office operating environments. One attempt to address this issue is illustrated in U.S. Pat. Nos. 5,541,722 and 5,557,394, which modify the design of the FIG. 3 apparatus by providing light emitting diode arrays as optical exposure units within a transparent drum that has a photoconductor formed on its surface. While this modification achieves reduced size, it requires the use of a transparent drum, which is generally undesirable for reasons of both cost and performance. Additionally, the design strategy of these two patents is limited in its potential for success because the modifications do nothing to reduce the space required by the developing units, which typically occupy a far larger volume than the optical exposure units.
It is thus desirable to provide an improved configuration for an electrophotographic apparatus. An associated, but distinct consideration in the design of high performance, low cost electrophotographic systems is the desirability of providing a more compact and higher performance developer. To appreciate this design consideration, it is useful to consider the design of a conventional developing unit used in some electrophotographic processes, illustrated in FIG. 4. The developing unit of FIG. 4 generally comprises a conductive roller that transports a developer consisting of a mixture of magnetic carrier particles and plastic or other toner ink particles to the surface of a photoconductive drum which carries a latent image. The developing unit includes roller, stirrers or other mechanisms to agitate the developer. The agitated developer adheres to the surface of the roller and a predetermined thickness of developer is maintained on the roller by use of a doctoring blade. The toner is triboelectrically charged and the toner and carrier are carried from a reservoir within the developing unit to a position adjacent the photoconductive drum by the roller as the roller rotates in an opposite sense to the photoconductive drum. A developing bias from a d.c. power supply and/or an a.c. power supply is applied between the photoconductive drum and the developing unit to form an electrical field that transports the developer from the roller to the photoconductive drum.
FIG. 4 shows an exemplary structure of a conventional developing unit in which a tribocharging blade 80 and a feed roller 82 are provided adjacent to a developing roller 84 within a housing 86. The tribocharging blade 80 is held against a surface of the developing roller 84, for example, by a spring mechanism. A stirrer 88 is provided within the reservoir of the developer mixture of carrier and toner 90. During developing, the toner 90 is agitated by the stirrer 88 and then distributed on a surface of the feed roller 82. Next, the toner 90 on the surface of the feed roller 82 is conveyed to the surface of the developing roller 84 by the feed roller. At the same time, a strong shearing force between the tribocharging blade 80 and the developing roller 84 causes the toner 90 to be tribocharged. Subsequently, the toner 40 is selectively transferred to a surface of a photoconductor 10 by the electrostatic mechanism discussed above, thereby achieving the developing operation.
In the conventional developing unit discussed above, there are a range of problems observed:
(1) The load caused by the frictional force between the tribocharging blade 80 and the developing roller 84 is large and the fluctuation in that load can be dramatic, resulting in variations in the charging of the toner 90.
(2) A nip between the tribocharging blade 80 and the developing roller 84 is small, introducing further instability to the tribocharging of the toner.
(3) Toner particles arc crushed by the strong shearing force between the tribocharging blade 80 and the developing roller 84, resulting in a fogging effect and reducing the image quality.
(4) The reservoir for storing the toner is separated from the other developing mechanisms. The configuration of the developing unit tends to be complicated, resulting in wasted space and in residual toner being left in the developing unit that cannot be accessed in developing operations.
It is therefore desirable for a developing unit to have a smaller volume, improved integration and improved performance as a part of an overall strategy to facilitate the production of a high performance, readily manufactured color electrophotographic printing apparatus.